Thermosetting resin compositions are employed for casting, impregnating, laminating, molding, insulation materials, structural materials, coating or adhesives. The compositions have strict requirements in physical properties and especially heat resistance and toughness have become more important. It, however, is very difficult to improve both heat resistance and toughness, because it is usual that the composition having high heat resistance has poor toughness and the composition having high toughness adversely has poor heat resistance.
It is also known that an article prepared from polyisocyanate and polyepoxide exhibits good heat properties. The cured article obtained from polyisocyanate and polyepoxide has good heat resistance but is stiff and fragile, especially poor in impact strength. It is believed that the heat resistance is given by oxazolidone groups and cyclic structures from isocyanurate groups which are produced when curing, and that the poor impact strength is given by highly crosslinked structure from trimerized isocyanurate groups.
The oxazolidone cyclizing reaction is conducted in the presence of a catalyst or at an elevated temperature in the absence of an catalyst. Typical catalysts are tertiary amines (e.g. imidazole, hexamethylenetetramine), quaternary ammonium salts (e.g. tetraethylammonium iodide), a complex of a Lewis acid and a base (e.g. aluminum chloride and pyrolidone, aluminum chloride and phosphate), lithium halide, a complex of lithium halide and phosphoric acid (e.g. lithium bromide and tributylphosphine oxide) and the like. As the catalyst, a complex of an organotin halide and a Lewis salt or stibonium is also proposed by Matsuda et al. in J. Org. Chem. pp 2177-2185, 51(12), 1986 and Chemistry Letters, pp 1963-1966, 1986. Japanese Kokoku Publication 500730/1987 to Marks Morris et al. further discloses an organoantimony iodide as a catalyst for preparing a polyoxazolidone resin.
Japanese Kokoku Publication 287126/1989 discloses a complex of an organotin halide and stibonium as a catalyst which provides a cured article having improved impact resistance and good heat resistance. The performance, e.g. improved impact resistance, is still insufficient and further improvements are required.
Since vinyl resins have good impact resistance and toughness, it is also proposed that the polyoxazolidone resin is mixed with the vinyl resins to improve impact resistance. However, the polyoxazolidone resin is poor solubility in solvent and it is difficult to mix it with the vinyl resin in liquid condition. Since the polyoxazolidone resin has poor compatibility with the vinyl resins, cold blending is also impossible.